When I bought a Roco BR18-201 I wasn't interested in sound and didn't want to pay more for the model with sound in it. But after seeing videos of the model and the real thing I regretted not having sound, because it's interesting from a three cylinder locomotive. I knew almost nothing about sound decoders and there is little information in English about the installation in this model. Recent research showed me enough to do the job. In case it is of use to anyone, I give some information here.

First, Roco uses Zimo decoders and the BR18-201 is already arranged to accept sound. There are two extra wires between the loco and tender for the loudspeaker. The circuit boards in the tender and locomotive have solder pads for connecting speaker to decoder. The Zimo 'Sugar Cube' speaker (shown at the bottom in the first photo) is supplied with a plastic sound chamber, but this is not needed. A chamber for the speaker is molded into the locomotive. The chamber measured 11x15 mm which is the size of standard Sugar Cube speaker.

The first photo also shows the extra weights from the tender and the original decoder which all were removed.

Zimo has a web page which lists sound files (http://www.zimo.at/web2010/sound/tableindex_EN.htm). For this locomotive there are free files from Roco and non-free files from Matthias Henning. On Henning's web site is a page of 'Tipps' which shows the Zimo MX645 decoder installed in the BR18-201. (https://www.henning-modellbahn.de/index.php?n1=4#to_4) Just now it also shows the installation of a MX647 decoder. I swear that second image was not there a couple weeks ago. Otherwise I would have used the MX647 because installation is so much simpler than with the MX645 which is what I used.

I got the Sugar Cube speaker and MX645 decoder from Streamlined Backshop in Indiana (https://store.sbs4dcc.com). Bryan Vianco, the owner, had decoders only with N. American sound files. He kindly installed the Roco file and sent off the decoder and speaker immediately. I have purchased other decoders from him in recent years and recommend him. His web site has videos and recommendations for installing Sugar Cube speakers.

The MX 645 decoder comes with a 6 pin plug but the wires are too long. I bought the version which has just the wires. I cut the wires to length and soldered them to a 6 pin plug. Two other wires connect a small capacitor which comes with the decoder. It supposedly improves low speed performance. Two more wires go to solder pads at the front for the speaker in the locomotive. Three function wires are unused. There is no spare space - it's a very tight fit. For this reason MX647 looks like a better choice.

On the test track the locomotive works well and sounds good. Research also suggests that instead of using the Zimo programmer, the Roco Z21 can be used to load sound files into Zimo decoders, and to update the decoders. There's a video showing it being done - https://www.youtube.com/watch?v=7fd058-ofHI It's not a very good video but it shows that the Z21 works. For the sake of learning about it, I'm tempted to try loading the Henning sound files. For the time being, the Roco sounds seem fine.